Methods and systems for predicting failures in x-ray tubes

ABSTRACT

The present approach relates to generating one or both of a failure prediction indication for an X-ray tube or a remaining useful life estimate for the X-ray tube. In one implementation, a trained static tube model is used in estimating health (e.g., thickness) of the electron emitter of the X-ray tube, which in turn may be used in predicting remaining useful life of an electron emitter of the X-ray tube.

BACKGROUND

The subject matter disclosed herein relates to X-ray tubes and, inparticular, to predicting X-ray tube failure.

Non-invasive imaging technologies allow images of the internalstructures or features of a subject (patient, manufactured good,baggage, package, or passenger) to be obtained non-invasively. Inparticular, such non-invasive imaging technologies rely on variousphysical principles, such as the differential transmission of X-raysthrough the target volume, to acquire data and to construct images orotherwise represent the internal features of the subject.

In such X-ray based non-invasive imaging contexts, X-ray tubes aretypically used to generate the X-rays passed through the subject.Examples of imaging systems employing X-ray tubes include, but are notlimited to systems for: radiography, mammography, tomosynthesis, C-armangiography, fluoroscopy, and computed tomography (CT) systems, as wellas others. The X-rays emitted by X-ray tubes in such systems aregenerated in response to control signals during an examination orimaging sequence.

Typically, the X-ray tube includes a cathode and an anode. An emitterwithin the cathode may emit a stream of electrons in response to heatresulting from an applied electrical current, and/or an electric fieldresulting from an applied voltage. The anode may include a target thatis impacted by the stream of electrons. The target may, as a result ofimpact by the electron beam, produce X-ray radiation to be emittedtoward an imaged volume.

In practice, such X-ray tubes have a finite useful life. However, evenX-ray tubes of the same type and model may vary as to their useful life.However, it may be difficult to estimate or predict the useful life ofan X-ray tube. As a result, X-ray tubes may either be proactivelychanged while useful life remains, or may be used until failure,resulting in unscheduled downtime that is inconvenient for patients andstaff.

BRIEF DESCRIPTION

In one embodiment, a method is provided for assessing health of an X-raytube. In accordance with this method, data points are acquired for arespective X-ray tube installed in an imaging system over time. The datapoints are processed using a trained X-ray tube model to calculate anestimated emitter drive current or estimated X-ray tube current overtime. A trending indicator is calculated over time based on theestimated emitter drive current or estimated X-ray tube current. Acombined indicator is calculated using the trending indicator. Anindication of X-ray tube health derived from the combined indicator isprovided.

In a further embodiment, a method is provided for training an X-ray tubemodel. In accordance with this method training data points are acquiredfor a respective X-ray tube after installation of the X-ray tube in animaging system. A regression model is calculated using the training datapoints to derive respective values for a plurality of coefficients. TheX-ray tube model is constructed using the plurality of coefficients. TheX-ray tube model returns an estimate of electron emitter drive currentor X-ray tube current in response to input data points.

In an additional embodiment, a processor-based system is provided. Inaccordance with this embodiment, the processor-based system comprises: amemory configured to store executable routines and a processingcomponent configured to execute the routines stored in the memory. Theroutines, when executed, cause acts to be performed comprising:acquiring data points related to operation of an X-ray tube over time;processing the data points using a trained X-ray tube model to calculatean estimated emitter drive current or estimated X-ray tube current overtime; calculating a trending indicator over time based on the estimatedemitter drive current or estimated X-ray tube current; calculating acombined indicator based on the trending indicator; and providing anindication of X-ray tube health derived from the combined indicator.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of an embodiment of a computedtomography (CT) system configured to acquire CT images of a patient andprocess the images in accordance with aspects of the present disclosure;

FIG. 2 illustrates a block diagram of an X-ray tube, in accordance withaspects of the present disclosure;

FIG. 3 depicts an example of a static X-ray tube model describingelectrical parameters and their interrelationship, in accordance withaspects of the present disclosure;

FIG. 4 depicts an example curve plotting an indicator of X-ray tubelife, in accordance with aspects of the present disclosure;

FIG. 5 depicts an example curve plotting a combined indicator of X-raytube life, in accordance with aspects of the present disclosure;

FIG. 6 depicts a flow chart illustrating creation and use of an X-raytube model used to assess X-ray tube life, in accordance with aspects ofthe present disclosure;

FIG. 7 depicts an example curve plotting combined indicators for avariety of X-ray tubes, in accordance with aspects of the presentdisclosure;

FIG. 8 depicts an example curve plotting linearized version of thecombined indicators of FIG. 7, in accordance with aspects of the presentdisclosure;

FIG. 9 depicts a flow chart illustrating creation and use of an X-raytube model used to assess X-ray tube life depicts an example curveplotting an indicator of X-ray tube life, in accordance with aspects ofthe present disclosure;

FIG. 10 depicts a plot of mA at 5 ms/average mA over a window of timeexhibiting an unstable mA behavior; and

FIG. 11 depicts the a trending indicator including the unstable mAbehavior exhibited in FIG. 10, in accordance with aspects of the presentdisclosure.

DETAILED DESCRIPTION

One or more specific embodiments will be described below. In an effortto provide a concise description of these embodiments, all features ofan actual implementation may not be described in the specification. Itshould be appreciated that in the development of any such actualimplementation, as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

When introducing elements of various embodiments of the presentinvention, the articles “a,” “an,” “the,” and “said” are intended tomean that there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.Furthermore, any numerical examples in the following discussion areintended to be non-limiting, and thus additional numerical values,ranges, and percentages are within the scope of the disclosedembodiments.

While the following discussion is generally provided in the context ofmedical imaging, it should be appreciated that the present techniquesare not limited to such medical contexts. Indeed, the provision ofexamples and explanations in such a medical context is only tofacilitate explanation by providing instances of real-worldimplementations and applications. However, the present approaches mayalso be utilized with respect to X-ray tubes in other contexts, such asX-ray tubes used in the non-destructive inspection of manufactured partsor goods (i.e., quality control or quality review applications), and/orthe non-invasive inspection of packages, boxes, luggage, and so forth(i.e., security or screening applications). In general, the presentapproach may be desirable in any imaging or screening context in whichan X-ray tube is utilized.

As discussed herein, an X-ray tube, such as those used in computedtomography (CT) imaging systems, may be subject to failure for variousreasons, including wear-out failures of the rotor (which rotates theanode from which X-rays are emitted) and/or the electron emitter (e.g.,a filament) (which emits electrons toward the anode). As used herein,the phrase “X-ray tube failure” may encompass instances in which anemitter of the X-ray tube has failed, even if other emitters arepresent, due to the loss of capacity of the X-ray tube. Hence, in thepresent context, the phrase X-ray tube failure may be understood toencompass a failure of an emitter in the X-ray tube, even if the X-raytube retains some X-ray generation capacity Such failures may beinconvenient for operators of imaging systems, particularly in instancesof unexpected failure of the X-ray tube while the scanner is being used.In accordance with the approach described herein, an algorithm isdisclosed that automatically predicts the failure of an X-ray tube withsufficient advance notice to allow a replacement part to be orderedand/or a service call to be scheduled and to allow patient exams to bescheduled appropriately such that disturbances to the examinations areminimized. In an alternative aspect, an algorithm may be provided inaddition or instead that provides an estimate of the remaining usefullife of the X-ray tube based on the deterioration of the electronemitter (referred to herein as an “emitter” or “electron emitter”).

With respect to emitter failures, as the emitter is used it evaporatesand is thereby reduced in thickness. As a result, lower emitter currentscan achieve the temperatures needed to drive a specified tube current ata specified voltage. By monitoring the evolution of the emitter currentneeded for a target tube current at any tube voltage, an estimate can beobtained of how far along an emitter is in its life cycle. With this inmind, the present approach is directed to predicting emitter failuresbefore they occur and to estimating the remaining life of the emitter atany stage within the life of the X-ray tube. This present approach isbased on modeling the temperature of an emitter, the evaporation rate ofthe emitter, and the current required to be transmitted through anemitter for a desired tube current and tube voltage setting. In oneaspect, the present algorithm can process all combinations of X-ray tubecurrent (mA) and X-ray tube voltage (kV) settings without needing tomonitor multiple indicator curves for different current and voltagesettings.

In one implementation, a prediction failure algorithm generates an alertonly close to the end of the life of the emitter. As a result, anoperator would not be actively aware of the health of the emitter untilthe near the end of the emitter useful life. To provide additionalinformation, in a further embodiment an indicator may be provided thatcontinuously informs the operator about the state of the emitter overtime. In one such approach, the respective algorithm may compute aremaining useful life of the emitter at a respective time.

Prior to discussing detailed aspects of the present approach however, anexample of an imaging system on which such an approach may be employedis described so as to provide useful context. With this in mind, FIG. 1illustrates an embodiment of an imaging system 10 for acquiring andimaging data using an X-ray tube in accordance with the approachesdiscussed herein. In the illustrated embodiment, system 10 is a computedtomography (CT) system designed to acquire X-ray projection data at oneor more energy spectra, to reconstruct the projection data intovolumetric reconstructions, and to process the image data for displayand analysis. The CT imaging system 10 includes one or more X-raysources 12, such as one or more X-ray tubes.

In certain implementations, the X-ray source 12 may be positionedproximate to a filter assembly or beam shaper 22 that may be used tosteer the X-ray beam 20, to define the shape and/or extent of ahigh-intensity region of the X-ray beam 20, to control or define theenergy profile of the X-ray beam 20, and/or to otherwise limit X-rayexposure on those portions of the patient 24 not within a region ofinterest. In practice, the filter assembly or beam shaper 22 may beincorporated within the gantry between the source 12 and the imagedvolume within patient 24.

The X-ray beam 20 passes into a region in which the subject (e.g., apatient 24) or object of interest (e.g., manufactured component,baggage, package, and so forth) is positioned. The subject attenuates atleast a portion of the X-rays 20, resulting in attenuated X-rays 26 thatimpact a detector array 28 formed by a plurality of detector elements(e.g., pixels) as discussed herein. Each detector element produces anelectrical signal that represents the energy deposition of incidentX-ray photons at the position of the detector element. Electricalsignals are acquired and processed to generate one or more projectiondatasets. In the depicted example, the detector 28 is coupled to thesystem controller 30, which commands acquisition of the digital signalsgenerated by the detector 28.

A system controller 30 commands operation of the imaging system 10 toexecute filtration, examination and/or calibration protocols, and toprocess the acquired data. With respect to the X-ray source 12, thesystem controller 30 furnishes power, focal spot location, controlsignals and so forth, for the X-ray examination sequences. In accordancewith certain embodiments, the system controller 30 may control operationof the filter assembly 22, the CT gantry (or other structural support towhich the X-ray source 12 and detector 28 are attached), and/or thetranslation and/or inclination of the patient support over the course ofan examination.

In addition, the system controller 30, via a motor controller 36, maycontrol operation of a linear positioning subsystem 32 and/or arotational subsystem 34 used to move components of the imaging system 10and/or the subject 24. The system controller 30 may includesignal-processing circuitry and associated memory circuitry. In suchembodiments, the memory circuitry may store programs, routines, and/orencoded algorithms executed by the system controller 30 to operate theimaging system 10, including the X-ray source 12 and/or filter assembly22, to process the digital measurements acquired by the detector 28,and/or to monitor and/or estimate X-ray tube emitter health or remaininglife in accordance with the steps and processes discussed herein. In oneembodiment, the system controller 30 may be implemented as all or partof a processor-based system.

The X-ray source 12 may be controlled by an X-ray controller 38contained within the system controller 30. The X-ray controller 38 maybe configured to provide power, timing signals, and/or focal spot sizeand locations to the X-ray source 12.

The system controller 30 may include a data acquisition system (DAS) 40.The DAS 40 receives data collected by readout electronics of thedetector 28, such as digital signals from the detector 28. The DAS 40may then convert and/or process the data for subsequent processing by aprocessor-based system, such as a computer 42. In certainimplementations discussed herein, circuitry within the detector 28 mayconvert analog signals of the photodetector to digital signals prior totransmission to the data acquisition system 40. The computer 42 mayinclude or communicate with one or more non-transitory memory devices 46that can store data processed by the computer 42, data to be processedby the computer 42, or instructions to be executed by image processingcircuitry 44 of the computer 42. For example, a processor of thecomputer 42 may execute one or more sets of instructions stored on thememory 46, which may be a memory of the computer 42, a memory of theprocessor, firmware, or a similar instantiation.

The computer 42 may also be adapted to control features enabled by thesystem controller 30 (i.e., scanning operations and data acquisition),such as in response to commands and scanning parameters provided by anoperator via an operator workstation 48. The system 10 may also includea display 50 coupled to the operator workstation 48 that allows theoperator to view relevant system data, imaging parameters, raw imagingdata, reconstructed data, basis material images, and/or alternativematerial decomposition images, and so forth. Additionally, the system 10may include a printer 52 coupled to the operator workstation 48 andconfigured to print any desired measurement results. The display 50 andthe printer 52 may also be connected to the computer 42 directly (asshown in FIG. 1) or via the operator workstation 48. Further, theoperator workstation 48 may include or be coupled to a picture archivingand communications system (PACS) 54. PACS 54 may be coupled to a remotesystem or client 56, radiology department information system (RIS),hospital information system (HIS) or to an internal or external network,so that others at different locations can gain access to the image data.

With the preceding discussion of an overall imaging system 10 in mind,FIG. 2 illustrates a block diagram of an X-ray source in the form ofX-ray tube 60, similar to an X-ray tube used in computed tomography (CT)machines. As illustrated, the X-ray tube 60 may include a cathode 62 andan anode assembly 64 encased in a housing 66. The anode assembly 64includes a rotor 68 that may turn an anode 70 (e.g., a rotating anodedisc). The anode 70 and the rotor 68 may rotate about a stationary shaft76.

During operation of the X-ray tube 60, the anode 70 emits an X-ray beam20 when struck by an electron beam 80 emitted by an electron emitterstructure (see FIG. 3, emitter 112) of the cathode 62. In some X-raytubes, electrostatic potential differences in excess of 20 kV arecreated between a cathode assembly 86 coupled to voltage source 82 andthe anode 70. As such, electrons may be emitted by the cathode assembly86 that accelerate towards the anode 70, which generates X-rays 20 inresponse to the incident electron stream.

The X-ray tube 60 is supported by the anode assembly 64 and the cathodeassembly 86, with the housing 66 defining an area of relatively lowpressure (e.g., a hermetically sealed vacuum enclosure). For example,the housing 66 may include glass, ceramics, stainless steel, or othersuitable materials. The anode 70 may be manufactured of any metal orcomposite, such as tungsten, molybdenum, copper, or any material thatcontributes to Bremsstrahlung radiation (e.g., deceleration radiation)when bombarded with electrons. The space between the cathode assembly 86and the anode 70 may be evacuated to minimize electron collisions withother atoms and to increase high voltage stability. Such evacuation mayadvantageously cause a magnetic flux to interact with (e.g., steer,focus) the electron beam 80.

As noted above with respect to FIG. 1, an X-ray controller 38 of thesystem controller 30 provides or controls electrical signals to theX-ray tube 60, including to the cathode assembly 86, that provide power,timing signals, and/or focal spot size and locations for the X-ray tube60 during operation. As such, the X-ray controller 38 has available datarelated to the electrical parameters used to operate the X-ray tube 60.Such electrical parameters may be maintained over time in an operatinglog or file that records parameters and electrical conditions for someor all exposure events associated with electron emission by the emitterof the cathode assembly 86. One example of such a log may be a log ofexposure information for the system acquired in real time and stored inlog files (or otherwise made available) for subsequent retrieval andanalysis.

In the depicted example, the X-ray controller 38 (or the overall systemcontroller 30 of which the X-ray controller is one aspect) may include acommunication component 90, a processor 92, a memory 94, a storage 96,input/output (I/O) ports 98, a display 100, and the like. Alternatively,some or all of these components may be shared with or may be thecomparable components of the computer 42 of the imaging system 10 thatadditionally handles processing of acquired data and interface withoperators and other connected devices. The communication component 90may be a wireless or wired communication line that may facilitatecommunication with various other processors, and the like. The processor92 may be any type of computer processor or microprocessor capable ofexecuting computer-executable code (e.g., computer-executableinstructions). The memory 94 and the storage 96 may be any suitablearticles of manufacture that can serve as media to storeprocessor-executable code, data, or the like. These articles ofmanufacture may represent computer-readable media (e.g., any suitableform of memory or storage) that may store the processor-executable codeused by the processor 92 to perform the presently disclosed techniques.

The memory 94 or the storage 96 may be used to store data downloaded viathe communication component 90. The memory 94 and the storage 96 may beused to store data received via the I/O ports 98, data analyzed by theprocessor 92, or the like. The memory 94 and the storage 96 may be usedto store data providing details regarding operational parameters for theX-ray tube 60, where if the data received via the I/O ports 98 and/ordata analyzed by the processor 92 does not satisfy (e.g., exceeds) anoperational parameter, the control system 38 may respond by adjusting anoperation of the X-ray tube 60 or a system that the X-ray tube 60 isassociated with.

The memory 94 or the storage 96 may also be used to store an application102, a firmware, software, or the like. The application 102, whenexecuted by the processor 92, may also enable the X-ray controller 38 orsystem controller 30 to assess health of an electron emitter of theX-ray tube 60 as discussed herein. The application 102, when executed bythe processor 92, may also enable the X-ray controller 38 or systemcontroller 30 to provide an alert or indication as discussed herein,such as with respect to the degradation of the emitter within thecathode assembly 86. It should be noted that the alert or indication maybe provided in any suitable manner (e.g., visual and/or audio alerts) toalert an operator or service personnel to an imminent emitter failure.

With the preceding in mind and to facilitate the subsequent discussion,a number of variables and terms related to the operation of the X-raytube 60 are provided here. Unless indicated otherwise, the X-ray tubeelectrical parameters or variables noted below may be obtained oracquired from a log or data store in which exposure data or parameters(e.g., the electoral X-ray tube operating parameters, scan and scannerdetails, and so forth) are acquired in real time (e.g., during a scan)and stored for later retrieval and analysis separate from a scan orexamination procedure. In particular, as used herein, variables relevantto pre-processing as discussed herein include: a code (i.e., anexamination number, from which one can infer exam number, from which onecan infer whether an exam is a regular or service exam), time (i.e., atime when generator log data message is sent), an exposure number (ExpNumber) (i.e., an exposure index for an exam), a focus selection(Select_Focus) (i.e., a focus size setpoint (e.g., 1 for small focus and2 for large focus)), a number of spits per exposure(Num_of_Spits_per_Exp), an examination prescription (Rx_Option) (i.e., atype of CT examination), an average current and/or voltage during anexposure, a current and/or voltage at or proximate to exposureinitiation (e.g., at approximately 5 ms), an emitter current and/orvoltage during a pre-heat phase, an emitter current and/or voltage atthe end of an exposure, a current setpoint (Select mA), a scan mode(e.g., single or multiple exposure, scan sequence, etc.), and so forth.

With the above parameters and variables in mind, pre-processing of X-raytube electrical data, as used herein, may include limiting the dataemployed in assessing emitter life to data points where tube current isgreater than a specified minimum, such as 0, tube voltage is within anacceptable range, the CT examination (Rx_Option) and/or scan mode are ofa suitable type (e.g., the emitter drive current stays constant for asuitable amount of time, such as for between 0.2 and 5 seconds), thenumber of spits per exposure is 0, the examination corresponds to themodel employed, and the actual X-ray tube current is close to thecommanded X-ray tube current, such as within 1%. Data points notmeetings these conditions may be discarded from the calculation of theindicator (discussed below) as part of data pre-processing, thusreducing the risk of false positive indications due to corrupt orun-representative measurements. It may be noted, however, that incertain implementations some or all of this data may be retained in theoverall exposure count used to determine the slope or other derivedparameters, also discussed in greater detail below.

The pre-processed (i.e., cleaned) X-ray tube electrical data points maybe provided as inputs to train or use an algorithm for predictingfailure or estimate the remaining useful life of an X-ray tube emitter112, as discussed herein. In one implementation, the present algorithmsare based on a static tube model 110 (FIG. 3) that is used to calculatea failure indicator or remaining life estimate based on the residuebetween the measured and the estimated emitter (e.g., filament) drivecurrent. In particular, given the static tube model shown in FIG. 3, theX-ray tube current I_(tube) can be characterized as a function ofemitter (e.g., filament) drive current I_(fil) and the X-ray tubevoltage V_(tube). One example of the derivation and parameterization ofsuch a model can be found in U.S. Pat. No. 7,023,960 issued Apr. 4, 2006to General Electric Company and titled “Method of adjusting the emissionrate of radiation from a source of radiation”, which is incorporatedherein in its entirety for all purposes. Though examples are generallydescribed below in which the estimated emitter drive current iscalculated in view of the X-ray tube current, it may be appreciated thatit is also contemplated that the estimated X-ray tube current mayinstead be calculated in view of the emitter drive current and that theestimated X-ray tube current could be used in the same manner as theestimated emitter drive current as described below. For simplicity onlythe emitter drive current implementation is discussed in detail below,though one skilled in the art will readily appreciate the applicabilityof the present discussion to a corresponding use based on an estimatedX-ray tube current.

Aspects of the model, such as the model coefficients, can be estimatedby fitting a model with the training data, which are usually collectedfrom start life of the X-ray tubes. The predicted emitter (e.g.,filament) drive current can then be calculated from the inverse statictube model. Typically, the X-ray tube current at a fixed X-ray tubevoltage is a function of the emitter temperature. As the emitter becomesthinner (i.e., evaporates) because of usage, the resistance of theemitter increases and thus the emitter drive current required to attaina target temperature becomes lower. In the case of a round cross-sectionemitter (e.g., a filament) this approximate proportionality relationshipbetween the emitter current, temperature T and filament diameter d maybe characterized by:

I _(fil) ² ∝T ³ d ³  (1)

The proportionality factor depends on the emitter emissivity, whichitself depends on the cathode geometry. Additionally, the initialemitter thickness (e.g., filament diameter) may also be subject tovariability.

With this in mind, if it is assumed that the temperature is constant,then the quantity I_(fil) ^(2/3) must be approximately proportional tothe diameter or other measure of thickness. A first order approximationis to assume that for fixed X-ray tube current and X-ray tube voltagesettings, the temperature remains constant throughout the life of theemitter. In practice, however, it has been observed that this is nottrue as the temperature of the emitter does change as the thickness(e.g., diameter) of the emitter decreases, and therefore theproportionality relationship between I_(fil) ^(2/3) and the emitterthickness is not accurate. A possible explanation for this might beobtained by noting that hot spots have been observed to develop on theemitter which leads to a non-uniform distribution of temperature on theemitter. Nevertheless, within the context of certain implementations ofthe present approach, it may be assumed that the general proportionalityrelation holds with the same constant of proportionality throughout thelife of an emitter. To account for variation, it is not assumed that theconstant of proportionality remains constant across emitters. Instead,for each emitter, the constant of proportionality is learned from theinitial portion of the life of the emitter and it is assumed that thisconstant remains constant throughout the life of the emitter.

In order to normalize the variation in emitter currents at differentkV−mA settings, certain implementations of the present approachnormalize to the current observed at the beginning of the life of theX-ray tube at each kV−mA setting. With this normalization, thefractional reduction in the quantity I_(fil) ^(2/3) may be used as aproxy to quantify the thinning of the emitter over time. In this manner,the life of a respective emitter at any age t may be represented as:

$\begin{matrix}{\frac{{I_{fil}^{2/3}(t)} - {I_{fil}^{2/3}(0)}}{I_{fil}^{2/3}(0)} \approx \frac{{d(t)} - {d(0)}}{d(0)}} & (2)\end{matrix}$

where the expression on the left hand side is evaluated at any fixedkV-mA setting. Based on this relationship, the fractional reduction inthe emitter current to the power ⅔ is equal to the fractional reductionin the diameter (or more generally thickness) of the emitter. Thisquantitative assessment of the reduction in emitter thickness may beused as a proxy of indicator of the wear on the respective X-ray tube.In one implementation, this indicator may be denoted as I^(rel)(t) wherethe superscript denotes that the indicator captures a relative change inthe filament current. This may be defined as:

$\begin{matrix}{{I^{rel}(t)}:=\frac{{I_{fil}^{2/3}(t)} - {I_{fil}^{2/3}(0)}}{I_{fil}^{2/3}(0)}} & (3)\end{matrix}$

It may be noted that in practice, the quantity I_(fil)(0) is notdirectly measured but is instead estimated using the emitter modelevaluated at the current kV-mA setting. As the emitter model, asdiscussed herein, is learned using data from the start of life of theX-ray tube emitter, the estimated quantity indeed corresponds to theemitter current that would have resulted on the fresh filament at theoperating kV−mA setting.

With the preceding in mind, in practice, the long-term feedback controlsystem implemented in the X-ray generator automatically adjusts theemitter drive current from one exposure to the next such that the X-raytube current remains close to the desired setting. Thus, the measuredemitter drive currents will be lower than the current predicted usingthe regression coefficients learned in the initial phase of the emitter.Assuming that constant tube current and tube voltage settingscorresponds to a constant temperature, it may be assumed that therelative reduction in the I_(fil) ^(2/3) should be proportional to therelative reduction in thickness. As a result, if the indicatorI^(rel)(t) of Equation 3 is plotted across the life time of the X-raytube against a measure of the total usage of the X-ray tube (e.g., thecumulative number of exposures that have been applied using the X-raytube) a downward-trending curve should be observed. As the emitterapproaches failure, the reduction in the emitter current accelerates. Asa result, a significant change of this indicator may be observed, interms of both the magnitude and the slope. An example of this indicatoris shown in FIG. 4. In this figure, the fraction of life (x-axis) at anypoint is measured as the ratio of the total number of exposures thathave occurred on the emitter to the total number of exposures on theemitter at failure.

As seen in FIG. 4, the downward trend in the value of I^(rel)(t)accelerates towards the end of life of the emitter life. Thus, the slopeof the indicator provides useful information that can be used to predictemitter failure. In accordance with the present approach, a combinedindicator may be employed that combines the value of the indicator withthe slope to produce a single indicator:

I ^(comb)(t)=I ^(rel)(t)+50000|I ^(rel)(t)|S ^(rel)(t)  (4)

where S^(rel) (t) is the slope or the rate of change of I^(rel) (t)relative to the exposure count. The proportionality factor of|I^(rel)(t)| is added to the slope term to emphasize that the slopeplays a more significant role in the indicator towards the end of lifeof the emitter at which point the factor |I^(rel)(t)| will be moresignificant. In addition, a factor of 50000 is employed in this examplebased on trial and error to ensure that the final combined indicatordoes sufficiently utilize the slope information without incurringunnecessary fluctuations. FIG. 5 illustrates the evolution of thisindicator as a function of the fraction of life.

Comparing FIGS. 4 and 5, it may be observed that the combined indicatordoes decrease at a faster rate towards the end of life of the emitterand reaches a lower value at the end. Thus, incorporating the slopeinformation into the indicator helps to improve the detectability ofimpending failures.

With respect to the generation and use of an emitter failure detectionalgorithm as used herein, an example of a process and use is provided.This example is provided as a two-step process, with the first steprelated to modeling the relationship between the emitter drive currentand the X-ray tube current and voltage and the second step related toapplying the constructed model to monitor incoming data and set alarmsfor potential failure.

With respect to the first step, an inverse static X-ray tube model(i_(fil)=f(v, i)) is built. Usable data is separated into two subsetsfor small and large emitters, respectively. The following steps areapplied separately to each emitter size subset (i.e., applied separatelyto the large emitter data and to the small emitter data), as the twoemitters can be used independently, and therefore they can age atdifferent speeds.

In this example, and with the above constraints in mind, training datapoints are then collected for linear regression model fitting. In orderto ensure the training set has sufficient data points, one approach isto use some threshold time window of data (e.g., the next 1-day, 7-daysor 31-days of data) as the training set after the tube installation. Ifthe number of training points is below a threshold (e.g., 1,000, 2,000,and so forth)), one more day of data is added to the training set untilthe number of training points is above this threshold. For a new X-raytube, one week of data may be sufficient to build the model.

The mA-kV settings used in the training set are counted. In oneembodiment, during the monitoring phase, described below) the algorithmwill not consider a new point for further analysis if its mA-kV settingdoes not appear in the training set.

The regression model i=f(v, i_(fil)) is calculated and used to estimatethe coefficients. Using these coefficients, the inverse static tubemodel is constructed. This model may, as discussed below, be used toestimate the emitter drive current by using the estimated coefficients.

Turning to FIG. 6, this first step is illustrated with respect to theupper branch of the depicted flow chart. As shown in this example, X-raytube generator data points 150 are collected over time. In this modeltraining example, some threshold number of points meeting one or morepre-processing criteria (such as those discussed above) are collectedfor use in training. In the depicted data point pre-processing or“cleaning” occurs at step 152, an output of which are generator datapoints meeting the established pre-processing criteria and suitable formodel building (step 154). As noted above, while data points may bediscarded for the purpose of indicator calculation, these points maystill be retained as part of the exposure count in the regression usedin determining a slope of the curve. Thus, as shown in FIG. 6, modelbuilding 154 is performed to generate a model 156 for a respective X-raytube 60. The model 156 may then be used in monitoring the respectiveX-ray tube 60 for imminent emitter failure or remaining useful lifeestimation, as discussed with respect to the second step and as shown inthe lower branch of the flow chart of FIG. 6.

With respect to the second step, additional generator data points 150are collected over time as the X-ray tube 60 is used. Based on new datapoints 150 and the model 156 (e.g., an inverse static X-ray tube model)the estimated emitter drive current is calculated. In this example, thei_(fil) from the inverse static tube model is the emitter drive current.

The trending indicator, I^(rel)(t), may be calculated as:

$\begin{matrix}{{I^{rel}(t)}:={\frac{I_{fil}^{2/3}(t)}{\left( I_{fil}^{est} \right)^{2/3}} - 1}} & (5)\end{matrix}$

where I_(fil)(t) denotes the filament current measured in a respectiveexposure indexed by t.

In one implementation, outliers in the measurements are identified andremoved. By way of example, measurements I^(rel)(t) may be used forslope calculation that meet one or more of criteria such as, but notlimited to: I^(rel)(t)<Mean(W)+1.0×Std(W),I^(rel)(t)>Mean(W)−1.0×Std(W), I^(rel)(t)<Mean(W)+2.0×Std(W),I^(rel)(t)>Mean(W)−2.0×Std(W), (mA_at_5 ms(t))/(Average_mA(t))<1.1,(mA_at_5 ms(t))/(Average_mA(t))>0.9,(mA_at_exposure_end)/(Average_mA(t))>0.95,(mA_at_exposure_end)/(Average_mA(t))<1.05, (Average_mA(t))/(CommandedmA)>0.95, (Average_mA(t))/(Commanded mA)<1.05,(mA_at_exposure_end)/(Commanded mA)>0.98,(mA_at_exposure_end)/(Commanded mA)<1.02, and so forth, where W is awindow containing the past 1000 points I^(rel)(t−1000), . . . ,I^(rel)(t−1). The purpose of certain of these criteria may be toalleviate the effect of mA unstable, identified as the abnormal behaviorfor mA_at_5 ms/Average_mA.

For missing data, the time gap between two exposures may be checked toensure it is above a threshold (e.g., 7 days, 31 days, and so forth). Ifa gap is detected, the moving window is reset at the first valid datapoint after the time gap.

In one implementation, the slope S^(rel)(t) of the indicator iscalculated using the valid observations in the moving window. The slopeis calculated by performing linear regression of I^(rel)(t) against t,which is the raw exposure count prior to outlier removal. In otherwords, t measures the count of the current measurement in the raw data,and not the count in the data remaining after outlier removal. In oneembodiment, data points generated during service exams are also includedin the raw exposure count used for slope calculation. Based on the slopeS^(rel)(t) of the indicator, the combined indicator, I^(comb)(t), iscomputed per Equation 4.

Turning back to the flow chart of FIG. 6, this second step isillustrated with respect to the lower branch of the depicted flow chart.As shown in this example, X-ray tube generator data points 150 arecollected over time and cleaned, (i.e., pre-processed) as discussedabove. The data points, in conjunction with the model 156 are then usedto calculate a trend indicator and combined indicator (step 160) asdiscussed above. If alarm triggering criteria are met (decision block162) an alarm is generated. If not data collection and processingcontinues over time.

Although the combined indicator of Equation 4 is a good indicator forpredicting emitter failure, the value of the combined indicator isessentially a fractional change in the emitter life which does not havea straightforward practical interpretation. To address this concern, alinearized version of this indicator may be employed that directlymeasures the fractional remaining life of the emitter.

By way of example, and turning to FIG. 7, combined indicators for avariety of emitters is depicted. As shown in the figure, each emitterrepresented tends to follow the same evolution as a function of thefractional life of the emitter. Based on this observation, a model maybe fitted to predict the fractional remaining life from the indicatorvalue. This may take the form of a linearized version of the indicator.In one such example, a linearized version of the indicator is obtainedby transforming the relative indicator. The transformation may be givenby:

I ^(tr)(l)=max{1−a(1−e ^(bl)),0}  (6)

where a and b are parameters that are learned during the training phase.As seen in FIG. 8, the transformation converts the value of theindicator to a fraction between 0 and 1. The result of thetransformation is a linear curve as shown in FIG. 8. In practice, thelinear transformation may be used directly on the combined indicator oron a smoothed version of this indicator depending on whether the goal isfailure prediction or remaining useful life estimation, both of whichare discussed in greater detail below.

With respect to estimating remaining useful life of an X-ray tube, thecombined indicator of I^(comb)(t) of Equation 4 may be subject tosignificant fluctuations due to inaccuracies in the model, theoccurrence of unfamiliar usage patterns, and outliers in the data. Hencea remaining useful life estimate that is directly based on thisindicator would be correspondingly noisy and subject to fluctuation.These concerns may be addressed by smoothing the indicator. Thus, forremaining useful life estimation, the transformation of Equation 6 maybe applied to a smoothed version of the combined indicator ofI^(comb)(t) given in Equation 4. In other words, the curves associatedwith a combined indicator, such as those shown in FIG. 7, may be modeledusing a model of the form of Equation 6.

In this approach, the algorithm used to learn the model is applied tothe combined indicator of Equation 4, and not to the relative indicatorof Equation 3. Another difference is in the calculation of thefractional remaining life used in the training phase. Instead, thefractional number of exposures left may be used as the fractionalremaining life due to, in almost all sites observed, the usage rateremaining constant over the entire life of the emitter. Hence, thefractional cumulative wear is approximately equal to the fractionalnumber of exposures. In view of this observation, there is little or nogain in choosing a computationally complex quantity, such as thecumulative wear, instead of simply using the exposure count. A furthervariation in implementation may be employing a weighted fit (in whichhigher weights are assigned to the errors at the end of life of theemitter) while performing the fit. This ensures that the fit is accuratetowards the end of life of the filament, which is the regime of interestfor failure prediction.

With the preceding in mind, in one example of an implementation stepssuch as the following may be performed to learn a model useful toestimating remaining useful life of an X-ray tube.

With respect to learning the model, for each failed emitter, use theavailable data to compute the combined trending indicator of Equation 4.The combined trend indicator so calculated, a median filter is appliedover a window of exposure (e.g., over 300 exposures) to generate asmoothed indicator. The smoothed indicator at any time is the median ofthe indicators measured during the past window (e.g., 300) of validexposures that pass the outlier removal test and the missing data testdiscussed herein. In this example, let S denote the smoothed indicatorobtained by computing the median of the past window of exposures (e.g.,300 exposures).

For each data-point the fractional remaining life, denoted R, may becomputed in accordance with:

$\begin{matrix}{R = \frac{\begin{matrix}{{{Number}\mspace{14mu} {of}\mspace{14mu} {exposures}\mspace{14mu} {at}\mspace{14mu} {failure}} -} \\{{Number}\mspace{14mu} {of}\mspace{14mu} {exposure}\mspace{14mu} {completed}\mspace{14mu} {so}\mspace{14mu} {far}}\end{matrix}}{{Number}\mspace{14mu} {of}\mspace{14mu} {exposures}\mspace{14mu} {at}\mspace{14mu} {failure}}} & (7)\end{matrix}$

The fractional remaining life after any exposure may be denoted as R.

A weighted non-linear regression of R against S is performed, assuming afunctional form of Equation 6, to obtain a non-linear fit of the form:

R≈max{1−a(1−e ^(bs)),0}  (8)

and the fitted values for parameters a and b are obtained. The weightassigned to the error at each data-point is proportional to (1−R)² inorder to emphasize the fit accuracy at values of R close to 0, orequivalently, at data-points close to the end of life of the filament.In one embodiment, in performing the regression only data-pointsfollowing the first 5000 exposures are used.

Once the model is learned values of parameters a and b are available.One aspect of this approach is that the model can be continuouslyupdated whenever data from more failed filaments become available. Theprocessed data from these filaments can also be added to the trainingset to improve the robustness and accuracy of the parameter estimates.

The model, once learned, can be used to predict the remaining usefullife of other emitters. The steps involved in predicting the remaininguseful life of an emitter, in one example, may include the following.For a current emitter of interest, available data for the respectiveemitter is used to calculate the combined trending indicator inaccordance with Equation 4. The normalized trend indicator so computedis smoothed, such as using a median filter over some window of exposures(e.g., 300 exposures). Thus, the smoothed indicator at any time is themedian of the indicators measured during the past window of exposures.The smoothed indicator obtained by computing the median of the pastwindow of exposures may be denoted S.

For each measurement of the smoothed indicator, the time difference iscalculated between the earliest and latest samples. This time differencemay be used in calculating the median. In certain implementations, athreshold may be imposed such that a time difference exceeding threshold(e.g., 25 calendar days) is deemed too great and no median is calculatedover that window due to lack of current or “fresh” data.

The computed value of S and relation shown in Equation 1 are used withthe learned values of the parameters a and b to compute the estimate ofthe remaining fractional life R as:

R ^(est)=max{1−_(a)(1−e ^(bs)),0}  (9)

In one implementation, the estimated value of R may be reporteddifferently depending on whether the model is undergoing training or isfully trained. For example, in such an implementation, a value of “1”(i.e., full life) may be reported while the model is undergoing trainingwhile the calculated value is instead returned when the model istrained.

As may be appreciated with respect to the preceding steps, the output ofthe above algorithm is the fraction of the remaining life of theemitter. In performing such estimation, an assumption that the usagerate of the emitter will remain substantially fixed or constant isimplicit. The output of the above algorithm can be transformed into theremaining life in absolute calendar time with additional informationsuch as the current age of the filament in calendar time.

Although the smoothed combined indicator S described above is useful asa remaining life estimator, it may not be as suitable for impendingfailure prediction due to smoothing of the indicator tending to averageout abrupt transitions in the indicator. In practice, abrupt drops inthe value of the combined indicator are common at the end of life of theemitter, which is one of the justifications for incorporating the slopeinto the combined indicator. Therefore, for failure prediction it may bemore useful to rely not on the smoothed indicator, but on a processedversion of the raw combined indicator of Equation 4.

With this in mind, a prediction algorithm for predicting emitter failureis summarized below. The described prediction algorithm includes twoaspects: modeling the relation between the emitter drive current andX-ray tube current and voltage, and applying the constructed model tomonitor incoming data and set alarms for potential failure. Note that,in one implementation, the data used with respect to the predictionalgorithm has undergone preprocessing steps as described above.

With respect to the steps, the trending indicator I^(rel)(t) of Equation5 and the combined indicator I^(comb)(t) of Equation 4 are computed. Theunsmoothed combined indicator is transformed to get an unsmoothedremaining fractional useful life estimate in accordance with:

R ^(rough)(t)=max{1−a(1−e ^(bI) ^(comb) ^((t))),0}  (10)

It may be noted that in this example the transform is performed on theunsmoothed combined indicator (rather than the smoothed indicator usedfor the remaining useful life estimate described above) so that abruptchanges may have an effect, as mentioned above. Alternatively, insteadof not applying any smoothing at all, a less aggressive smoothing (e.g.using a lower number of exposures for the window used for mediancalculation) may be applied.

The number of remaining days of useful life of the respective emittermay be estimated as:

$\begin{matrix}{{D(t)} = {\frac{N(t)}{1 - {R^{rough}(t)}} - {N(t)}}} & (11)\end{matrix}$

where N(t) is the total age of the filament obtained by calculating thedifference between the current date stamp and the X-ray tubeinstallation date.

For pending or imminent emitter failure notification, one or more alarmcriteria may be established. For example, in one embodiment, an alarm ornotification may be generated when the following condition is met:

max{D(t),D(t−1),D(t−2),D(t−3),D(t−4)}<Thr  (12)

where Thr is a configurable threshold that represents the lead timeprior to emitter failure for which a notification is desired. In thisexample implementation, the triggering criteria requires 5 consecutivepoints that meet the threshold, which helps alleviate the effects ofoutliers.

It may be noted that, in an implementation where both remaining usefullife estimation and failure prediction are performed, the abovesequences of steps may be modified to facilitate performing bothalgorithms. For example, in the implementation of the remaining usefullifetime estimate described above, smoothing of the combined indicatorwas described as being performed prior to applying the non-lineartransform of Equation 6.

However, the smoothing operation is performed using a median filter, andthus there would be no significant difference in the final estimator ifthe smoothing was instead performed after the non-linear transformation.Therefore, in an alternative implementation, the non-linear transformmay be performed first on the unsmoothed combined indicator so that theresult of the non-linear transform operation can be used for both theremaining useful life estimate as well as the alarm generation (i.e.,failure prediction) aspect. Adopting this sequence of operations wouldobviate the need to repeat the non-linear transform on the smoothedsignal while computing the remaining useful life estimate.

An illustration of the proposed pipeline for performing all the stepsneeded for training the model, for remaining useful life estimation, andfor alarm generation is provided in FIG. 9. In this example, datacharacterizing the electrical operating parameters of each exposureevent are acquired, such as at depicted Step 200 at which log data isread from a log file. At Step 202, the acquired log data may be cleanedas discussed herein to remove data points deemed not representative orappropriate for the purposes of modeling or evaluation using a model.

In the depicted example, a determination (decision block 206) is made asto whether the X-ray tube 60 in question is a new X-ray tube. As notedherein, this may be based upon whether data exists for the tube for somethreshold number of days (e.g., seven days) or based upon othercriteria, such as the presence of a new tube indicator or flag in therespective database, the presence or absence of a model associated withthe X-ray tube and so forth. If a new X-ray tube 60 is determined to bepresent, a training mode of the depicted pipeline is set to ON (Step208). If the X-ray tube 60 is not new, a trained model exists and newdata points will be processed by the existing model for remaining lifeestimation and/or emitter failure prediction.

In the depicted implementation, once the new tube detectiondetermination is made the acquired data points may be processed toremove data points associated with unreliable exposure events (Step212), unstable mA, and so forth). Based on whether the model trainingmode is set to ON or OFF (decision block 216) one of two paths may befollowed. In the event the model training mode is set to ON, exposureparameters based on the data points are stored or memorized (Step 220)until a threshold amount of data is acquired (decision block 222). Oncethe threshold amount of data is available, the training mode is set toOFF (Step 226), and the model calculated (Step 228) as discussed herein.

Returning to Step 216, when the model training mode is set to OFF, theacquired data points are processed to remove outliers (Step 240), suchas based upon an absolute or statistical threshold. An emitter (e.g.,filament) indicator is calculated (Step 242) for each exposure event.The indicator may then be used in one or both of calculating a remaininglife percentage of the emitter (Step 250) or calculating a remainingnumber of days (Step 252) of the emitter. In the context of calculatinga remaining life percentage of the emitter, as discussed above, asmoothing Step 254 may be performed before performing the calculation.In the context of calculating a remaining number of days (Step 252) ofthe emitter, the calculated number of days may be used to calculate(Step 260) whether one or more trigger conditions are met for generatinga notification or alarm of pending emitter failure within a configuredtime frame.

In a further aspect, it may be appreciated that in addition to theremaining life and or emitter failure prediction information that may bederived in accordance with the present approach, other information mayalso be derived. By way of example, and turning to FIG. 10 and FIG. 11,it may be seen that information regarding an unstable mA condition mayalso be ascertained. For example, referring to FIG. 10, the ratiobetween mA at 5 ms and average mA of the X-ray tube in question becomesvery unstable starting early 2012. Correspondingly, turning to FIG. 11,it may be observed that the curve of the trending indicator is no longersmooth once the unstable mA condition is exhibited. Hence, it may bepossible to use the present trending indicator approach to also identifyother X-ray tube issues such as unstable mA, in addition to the otherconditions discussed herein. Unstable mA may be caused by issues thatare not related to the emitter itself, such as fluctuating pressureinside the X-ray tube, which in turn may be caused by outgassing or byvacuum leaks.

Technical effects of the invention include generating one or both of afailure prediction indication for an X-ray tube or a remaining usefullife estimate for the X-ray tube. In one implementation, a trainedstatic tube model is used in estimating health (e.g., thickness) of theelectron emitter of the X-ray tube, which in turn may be used inpredicting failure of the tube and/or estimating remaining useful lifeof the tube.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. A method for assessing health of an X-ray tube, comprising: acquiringdata points for a respective X-ray tube installed in an imaging systemover time; processing the data points using a trained X-ray tube modelto calculate an estimated emitter drive current or estimated X-ray tubecurrent over time; calculating a trending indicator over time based onthe estimated emitter drive current or estimated X-ray tube current;calculating a combined indicator using the trending indicator; andproviding an indication of X-ray tube health derived from the combinedindicator.
 2. The method of claim 1, further comprising: calculating aslope of the trending indicator over time; wherein the combinedindicator is calculated using the trending indicator and the slope ofthe trending indicator.
 3. The method of claim 1, wherein providing theindication comprises: evaluating the combined indicator over timeagainst one or more failure criteria; and generating a failurenotification in response to a respective failure criterion being met bythe combined indicator.
 4. The method of claim 3, wherein the failurenotification comprises a number of days until estimated X-ray tubefailure.
 5. The method of claim 1, wherein providing the indicationcomprises: applying a transformation to the combined indicator or asmoothed version of the combined indicator; and based on thetransformation, providing an indication of the remaining life of theX-ray tube.
 6. The method of claim 1, further comprising: identifyingand removing outliers in the trending indicator prior to performingslope calculation.
 7. The method of claim 2, wherein calculating theslope comprises performing linear regression of the tending indicatoragainst time.
 8. A method for training an X-ray tube model, comprising:acquiring training data points for a respective X-ray tube afterinstallation of the X-ray tube in an imaging system; calculating aregression model using the training data points to derive respectivevalues for a plurality of coefficients; and constructing the X-ray tubemodel using the plurality of coefficients, wherein the X-ray tube modelreturns an estimate of electron emitter drive current or X-ray tubecurrent in response to input data points.
 9. The method of claim 8,wherein the X-ray tube model comprises a model relating X-ray tubevoltage and electron emitter current with X-ray tube current.
 10. Themethod of claim 8, wherein acquiring data points comprises discarding aninitial portion of the data points.
 11. The method of claim 8, whereinthe X-ray tube model is constructed separately for each size of emitter.12. The method of claim 8, wherein the training data points arecharacterized by mA-kV settings for each respective training data point.13. A processor-based system, comprising: a memory configured to storeexecutable routines; and a processing component configured to executethe routines stored in the memory, wherein the routines, when executed,cause acts to be performed comprising: acquiring data points related tooperation of an X-ray tube over time; processing the data points using atrained X-ray tube model to calculate an estimated emitter drive currentor estimated X-ray tube current over time; calculating a trendingindicator over time based on the estimated emitter drive current orestimated X-ray tube current; calculating a combined indicator based onthe trending indicator; and providing an indication of X-ray tube healthderived from the combined indicator.
 14. The processor-based system ofclaim 13, wherein providing the indication comprises: evaluating thecombined indicator over time against one or more failure criteria; andgenerating a failure notification in response to a respective failurecriterion being met by the combined indicator.
 15. The processor-basedsystem of claim 14, wherein the failure notification comprises a numberof days until estimated X-ray tube failure
 16. The processor-basedsystem of claim 13, wherein providing the indication comprises: applyinga transformation to the combined indicator or a smoothed version of thecombined indicator; and based on the transformation, providing anindication of the remaining life of the X-ray tube.
 17. Theprocessor-based system of claim 13, wherein the routines, when executed,cause acts to be performed further comprising: calculating a slope ofthe trending indicator over time; wherein the combined indicator iscalculated using the trending indicator and the slope of the trendingindicator.
 18. The processor-based system of claim 17, whereincalculating the slope comprises performing linear regression of thetending indicator against time.
 19. The processor-based system of claim13, wherein the routines, when executed, cause acts to be performedfurther comprising: identifying and removing outliers in the trendingindicator prior to performing slope calculation.
 20. The processor-basedsystem of claim 13, wherein one or both of the memory and processingcomponent are provided in an X-ray controller or system controller ofthe X-ray tube based imaging system.